# Bug in the money gem? **URL:** \ **Category:** Ruby\ **Created:** [September 25, 2008, 8:36pm UTC](https://www.ruby-forum.com/t/bug-in-the-money-gem/149529 "2008-09-25T20:36:04Z")\ **Posts on this page:** 4\ **Page:** 1
**Author:** ![Eric\_LIn](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/45deac/32.png) [@Eric\_LIn](https://www.ruby-forum.com/u/Eric_LIn)\ **Post date:** [September 25, 2008, 8:36pm UTC](https://www.ruby-forum.com/t/bug-in-the-money-gem/149529/1 "2008-09-25T20:36:04Z")
For anyone of you using the money gem ([http://dist.leetsoft.com/api/](http://dist.leetsoft.com/api/) money/), I noticed today that 0 - 100 does not equal -100, it equals 100 instead (all numbers here represent Money objects). Do any of you find that odd? Eric ---
**Author:** ![Eric\_LIn](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/45deac/32.png) [@Eric\_LIn](https://www.ruby-forum.com/u/Eric_LIn)\ **Post date:** [September 25, 2008, 9:05pm UTC](https://www.ruby-forum.com/t/bug-in-the-money-gem/149529/2 "2008-09-25T21:05:31Z")
On Thu, Sep 25, 2008 at 2:34 PM, Eric LIn [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > For anyone of you using the money gem ([Too-biased](http://dist.leetsoft.com/api/) > money/), I noticed today that 0 - 100 does not equal -100, it equals > 100 instead (all numbers here represent Money objects). Do any of you > find that odd? I’m not sure, but it seems a bit strange. Here is the offending code: ``` # File lib/money/money.rb, line 79 ``` 79: def -(other\_money) 80: return other\_money.dup if cents.zero? 81: return dup if other\_money.cents.zero? 82: 83: if self.cents == 0 or currency == other\_money.currency 84: Money.new(cents - other\_money.cents, other\_money.currency) 85: else 86: Money.new(cents - other\_money.exchange\_to(currency).cents, currency) 87: end 88: end I think the first return would make sense if the other\_money variable had a negative value associated with it, however, other\_money’s value is not negative. HTH, Michael G. ---
**Author:** ![Eric\_LIn](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/45deac/32.png) [@Eric\_LIn](https://www.ruby-forum.com/u/Eric_LIn)\ **Post date:** [September 25, 2008, 8:49pm UTC](https://www.ruby-forum.com/t/bug-in-the-money-gem/149529/3 "2008-09-25T20:49:38Z")
On Sep 25, 2:32 pm, Eric LIn [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > For anyone of you using the money gem ([Too-biased](http://dist.leetsoft.com/api/) > money/), I noticed today that 0 - 100 does not equal -100, it equals > 100 instead (all numbers here represent Money objects). Do any of you > find that odd? Yes, it’s odd. The problem is line 80: # File lib/money/money.rb, line 79 79: def -(other\_money) 80: return other\_money.dup if cents.zero? 81: return dup if other\_money.cents.zero? 82: 83: if self.cents == 0 or currency == other\_money.currency 84: Money.new(cents - other\_money.cents, other\_money.currency) 85: else 86: Money.new(cents - other\_money.exchange\_to(currency).cents, currency) 87: end 88: end Try commenting line 80 out. I don’t see any reason why the class wouldn’t handle negative values, though I didn’t look carefully. – Mark. ---
**Author:** ![Eric\_LIn](https://www.ruby-forum.com/letter_avatar_proxy/v4/letter/e/45deac/32.png) [@Eric\_LIn](https://www.ruby-forum.com/u/Eric_LIn)\ **Post date:** [October 1, 2008, 10:50pm UTC](https://www.ruby-forum.com/t/bug-in-the-money-gem/149529/4 "2008-10-01T22:50:56Z")
On Sep 25, 12:04 pm, Michael G. [removed\_email\_address@domain.invalid](mailto:removed_email_address@domain.invalid) wrote: > 80: return other\_money.dup if cents.zero? > had a negative value associated with it, however, other\_money’s value > is not negative. > > HTH, > Michael G. Thanks for the response, I figured that’s the problem too. It seems the author made the mistake unintentionally, when he copies that line of code from the “+” method. Thanks for confirming my suspicion; I was thinking maybe it’s something in currency arithmetic that I don’t understand. I’ve emailed the author, but so far no response.